This invention relates to a writing medium to be filled in a ball point writing instrument and, more particularly, to a writing medium which can be erased by merely mechanically rubbing with an ordinary pencil eraser after writing, which can form a drawing line incorporating high adhering strength onto the surface of paper even immediately after writing on the paper with high solidifying velocity on the surface of paper, and which also incorporates excellent ageing stability.
There were heretofore as a writing medium or ink for a ball point writing instrument or a ball point pen, "liquid lead ink" which contained graphite and carbon black together with oily ingredients, "natural rubber-containing ink" which contained natural rubber ingredient suspended in composition ingredient together with pigments, and "rubber ingredient-containing ink" which contained, as disclosed, for example, in Japanese Patent Laid-open No. 145731/1979, rubber ingredient suspended in composition ingredient together with pigment. Since the former ink impregnates oily ingredient in a drawing line formed on the surface of paper into the interior of the paper together with the fine particles of graphite or carbon black as colorants, it cannot sufficiently perform its original purpose of obtaining erasable drawing line. Though the latter ink can obtain an erasable drawing line to some degree, it is not yet complete and further since it incorporates slow solidifying velocity on the surface of paper, it accommodates low adhering strength onto the surface of paper immediately after writing. It has accordingly such disadvantages and drawbacks that, if another paper is superimposed on the written surface of paper immediately after writing, if writer's hand is contacted with the drawing line immediately after writing or if the surface of paper is rubbed, the colorant forming the drawing line is readily transferred to the other part on the surface of paper thereby contaminating the surface of paper. In addition, inasmuch as the dissolution stability of added rubber ingredient to a solvent is low, the conventional ink has such disadvantage as low ageing stability in itself.